^^^ With what was said above by MrNorwall...I don't think thats the best way, this idea is not to come off as a "car sales man" they have the reputation of just closing the deal no matter what. An alternative way of doing what MrNorwall said, wait for them to wash their car...everyone washes their car when its nice out, its the american thing to do...so when they are out there washing, kindly walk over and first compliment them on their car. When you give them compliments it opens the doors to communcation. Thats the objective, communicate, dont be pushy...offer a couple tips here and there about washing, then let them know you do this for a living and you take pride in what you do. If you help them first, they will remember that, so when the time comes around and they dont know how to do something, they will consult your services.
This has worked numerous times for me in this business and other occupations im in. Be helpful and kind...it goes a long way! just my .02!